In general, a system for controlling a vehiclular generator includes a voltage controlling device having an external voltage sensing terminal. When a key switch of a vehicle is closed, a power transistor is made conductive by flowing of a base current, resulting in a field current flowing through a field coil, thereby enabling an AC generator (referred to as simply “generator” hereinafter) to output a generator voltage. Further, a constant-voltage power supply is produced from a battery voltage and a reference voltage is generated by the constant-voltage power source.
When the generator starts to generate electric power due to starting of an in-vehicle engine, the voltage controlling device operates so that the battery voltage to be detected by the external voltage sensing terminal exceeds the reference voltage to make a transistor 103 conductive and to turn off a power transistor 101, whereby the field current reduces and the generator output voltage of the generator lowers. In addition, when the battery voltage becomes lower, the power transistor is made conductive, and therefore the field current increases to raise the generator output voltage of the generator.
Furthermore, for example, Japanese Pat. No. 3102981 discloses a technology that an adjustment voltage varies in accordance with a duty signal from an external control unit related to a voltage controlling device.